1. Field of the Invention
The present invention relates to reading data from a disk in general and to a method and apparatus comprising reading data from a disk having missing or unreadable field address marks in particular.
2. Description of Prior Art
Disks used for storing data comprise a plurality of concentric tracks. Typically, each of the tracks comprises an index mark and is divided into a plurality of sectors, e.g. 0, 1, 2, 3 . . . n. Each of the sectors comprises a plurality of fields including an identification field and a data field. Each of the identification and data fields comprises an address mark.
In practice, the location of a sector on a disk need not correspond physically to the number used to identify it. For example, a sector identified as sector 5 may be physically the third sector from the index mark. To distinguish the two types of sector designations herein, the former is called a logical sector and the latter is called a physical sector. Thus, in the given example, logical sector 5 is physical sector 3.
When a particular logical sector is selected, the address mark associated with each field in the sectors is used as a control signal for commencing the reading of the selected sector. For example, if it is desired to read the data in logical sector 5 of a particular track, control circuitry in a disk controller detects each of the address marks and identification fields as they pass under a read head. When the control circuitry detects the address mark and reads the identification field associated with the logical sector 5, it searches for the next address mark associated with the data field and reads the data field.
Since the reading of data from a selected logical sector ordinarily depends on the presence and readability of two address marks in each logical sector, the failure of the control circuitry to detect one or more of the marks in any of the logical sectors can result in an inability to read a selected logical sector or a reading of the wrong logical sector. When this occurred in the past, it was necessary to use other methods to read a selected logical sector.
Heretofore, one of the methods used to recover data from a selected logical sector when one or more address marks on a track had been rendered unreadable comprised using a map of the location of the logical sectors on a disk to identify the physical sector corresponding to the selected logical sector and a counter.
In practice, the counter was loaded with the location of the physical sector, started by the index mark and incremented at a rate corresponding to the angular velocity of the disk, also called disk spindle speed. When the counter reached a predetermined count corresponding to the selected physical sector, an attempt was made to read data from the disk for a predetermined period corresponding to the length of the data field therein.
The foregoing described use of a counter to recover data was sufficient so long as the angular velocity of the disk was constant. Frequently, it was possible to miss the correct field (ID or data) and read the wrong field.
Another method which has been used for recovering data from a disk with unreadable field address marks has involved reading all of the data from a track and thereafter, by visual inspection, attempting to identify the selected data. In practice, however, this method is often impracticable.